Photonic Crystals
Photonic crystals interact strongly with electromagnetic radiation, when the periodicity of the photonic crystal corresponds to the scale of the wavelength of the electromagnetic radiation. The periodicity of the photonic crystal is achieved through a periodic modulation of the dielectric function. Due to the periodicity, photonic crystals display a photonic band structure with a fundamental stop-band as predominant feature for certain directions (see E. Yablonovitch, “Inhibited spontaneous emission in solid-state physics and electronics” Phys. Rev. Lett. 1987, 58, 2059; S. John, “Strong localization of photons in certain disordered dielectric superlattices” Phys. Rev. Lett. 1987, 58, 2486).
The promise for technological application in the optical properties of photonic crystals has sparked an enormous interest in methods of fabrication of photonic crystals. The main focus in future applications is directed towards the production of an all optical chip and integrated detector materials for optical sensing (see J. D. Joannopoulos, P. R. Villeneuve, S. Fan, “Photonic crystals: putting a new twist on light” Nature 1997, 386, 143; G. A. Ozin, S. M. Yang, “The race for the photonic chip: colloidal crystal assembly in silicon wafers” Adv. Funct. Mater. 2001, 11, 95; A. Arsenault, S. Fournier-Bidoz, B. Hatton, H. Miguez, N. Tetreault, E. Vekris, S. Wong, M. Y. San, V. Kitaev, G. A. Ozin, G. “Towards the synthetic all-optical computer: science fiction or reality?” J. Mater. Chem. 2004, 14, 781).
One common approach to photonic crystals is the self-assembly of spherical colloidal particles. Several methods have been developed to achieve colloidal crystal (often called “opal” due to the similarity with the gem structure) films of controlled thickness and various degrees of uniformity from spheres of varying diameters and materials (see P. Jiang, J. F. Bertone, K. S. Hwang, V. L. Colvin “Single-Crystal Colloidal Multilayers of Controlled Thickness” Chem. Mater. 1999, 11, 2132; S. Wong, S. Kitaev, S., G. A. Ozin “Colloidal Crystal Films: Advances in Universality and Perfection” J. Am. Chem. Soc. 2003, 125, 15589).
Infiltrating a photonic crystal with any other material in the form of a solid, gel, liquid, or gas causes a change in the photonic band structure and results in a shift of the fundamental stop-band (see R. C. Schroden, M. Al-Daous, C. F. Blanford, A. Stein, “Optical Properties of Inverse Opal Photonic Crystals” Chem. Mater. 2002, 14, 3305; C. F. Blanford, R. C. Schroden, M. Al-Daous, A. Stein, “Tuning solvent-dependent color changes of three-dimensionally ordered macroporous (3DOM) materials through compositional and geometric modifications” Adv. Mater. 2001, 13, 26; K. Yoshino, K. Tada, M. Ozaki, A. A. Zakhidov, R. H. Baughman, “The optical properties of porous opal crystals infiltrated with organic molecules” Japanese Journal of Applied Physics, Part 2: Letters 1997, 36(6A), L714).
For a non-swelling and inert colloidal photonic crystal material, its lattice parameters and structure will remain unchanged upon infiltration with a liquid. The values of refractive indices and refractive index changes can thus be measured via spectra taken from infiltrated colloidal crystals. The limiting factors are the structural or optical quality and uniformity of the photonic crystal and the resolution of the measuring instrument.
Colloidal Crystal Sensor Arrays
Colloidal particles can be assembled and embedded in a chemically responsive matrix, usually a gel incorporating ligands or binding sites selective to specific chemical moieties. Especially cross-linked colloidal crystal arrays have been extensively researched with regard to their potential applications as chemical sensors. The gels swell either in the presence of a liquid or swell selectively in the presence of a chemical moiety in that liquid or solvent. The lattice parameter changes due to the swelling and the optical behavior can be spectroscopically monitored (see J. H. Holtz; S. A. Asher, “Polymerized colloidal crystal hydrogel films as intelligent chemical sensing materials” Nature 1997, 389, 829; E. Reese, M. E. Baltusavich, J. P. Keim, S. A. Asher “Development of an intelligent polymerized crystalline colloidal array colorimetric reagent” Anal. Chem. 2001, 73, 5038).
Thus, the presence of a swelling liquid or a specific chemical entity can be detected by a shift in the band structure. It is problematic though to accurately determine refractive index variations as not only the index of the infiltrating liquid changes but also the refractive index of the colloidal crystal array changes, as the distance between spherical particles or void spaces is subject to change. This change is also non-linear as the swelling of the gel matrix is limited by the degree of cross-linking. The detection of rapid changes of solvent or mobile phase mixtures is also limited by diffusion and inertia or response time of the gel. These problems do not arise in the invention disclosed herein as the spheres of the photonic crystal column or wall materials used to produce the inverted crystal column do not react physically or chemically with the mobile phase.
Colloidal Crystal Arrays and Entropic Entrapping Chromatography
Arrays of air-spheres, void spaces or water-spheres embedded in a hydrogel displaying light diffraction have been manufactured and claimed for chromatographic separation applications (see Hydrogels with crystalline colloidal array of water voids for macromolecule separations and detection, Asher, Sanford A.; Liu, Lei. (University of Pittsburgh, USA). PCT Int. Appl. (2000), 48 pp. CODEN: PIXXD2 WO 2000000278 A1 20000106; Liu L., Li P., Asher S. A. Entropic trapping of macromolecules by mesoscopic periodic voids in a polymer hydrogel. Nature (1999 Jan. 14), 397(6715), 141-4).
The invention in the above patent by Asher et al. is restricted to macromolecules with linear chains that can assume one of a number of shapes from spherical to rod-like including a conformation with a maximum of conformational entropy. The rod-like or elongated shapes diffuse through the gel matrix into the isolated water-voids there to be entrapped after assuming an entropically favorable shape. Additionally, for the hydrogel to selectively entrap by an entropic effect, molecules of one certain average mass, equilibrium times of three to ten days are required.
After macromolecules of one certain average mass have been trapped, they have to partition back into a mobile phase, which will also take considerable time. The size of the template spheres has to be tailored for macromolecules of one specific average mass. One the other hand, the invention disclosed herein achieves separation by molecule-surface interaction of the materials to be separated contained in a mobile phase and the surface of the photonic colloidal crystal column or inverse photonic crystal column, not by entropic trapping. Thus there is no restriction regarding shape and size of molecules that can be separated.
While the molecules to be separated have to permeate through the hydrogel to the water voids, all mass transport in the present invention is achieved by a continuous mobile phase motion through the interstitial voids or connecting pores. Open mesopores enhance the separation but do not participate in the mass transfer. The present invention avoids the problems associated with hydrogel swelling, which would interfere with the monitoring of the spectral properties. The present invention is also advantageous in that the void and pore dimensions are invariant to solvent and temperature changes.
In the prior art, water void arrays of hydrogels have not been produced in capillaries, where the contact area between the confined hydrogel and a macromolecule solution is much smaller than with described hydrogel films and a macromolecule solution, and hence the partitioning would take much longer (see Hydrogels with crystalline colloidal array of water voids for macromolecule separations and detection. Asher, Sanford A.; Liu, Lei. (University of Pittsburgh, USA). PCT Int. Appl. (2000), 48 pp. CODEN: PIXXD2 WO 2000000278 A1 20000106; Liu L., Li P., Asher S. A. Entropic trapping of macromolecules by mesoscopic periodic voids in a polymer hydrogel. Nature (1999 Jan. 14), 397(6715), 141-4).
Monolithic Columns Made from Colloidal Crystals
The structural integrity is a prerequisite for future applications of photonic crystal materials and stationary phases. An isotropic monolithic material ensures uniform optical quality and consistent separation properties. This is achieved in the invention disclosed herein and has not been realized so far using other approaches.
Colloidal Crystals in Capillaries
Until recently, capillaries have been only completely filled with colloidal crystals by sedimentation, a process that requires weeks or months for completion if not assisted by gravity (centrifugation). The resulting capillary photonic crystals produced are not of the highest quality, especially using centrifugation, and their optical properties have not been investigated thoroughly with respect to axial and rotational uniformity. Due to the time requirements it is prohibitive to produce photonic crystal capillaries in the centimeter range by this method. (see W. L. Vos, R. Sprik, A. van Blaaderen, A. Imhof, A. Lagendijk, G. H. Wegdam “Strong effects of photonic band structures on the diffraction of colloidal crystals” Phys. Rev. B: Condens. Matter 1996, 53, 16231).
Colloidal crystal surface coatings have been produced in capillaries, which requires a meticulous control of the meniscus of the colloidal dispersion. A colloidal dispersion is pressed into a capillary, where the capillary rests in a temperature regulated environment. The meniscus and thus the microsphere deposition is controlled by adjusting the evaporation rates via temperature and by controlling the velocity of the dispersion liquid. These surface coating films presented in prior art are mostly monolayers of microspheres, or a continuous multi-layer coatings, but complete filling has not been achieved by this method. Additionally, high film quality or the order of the colloidal particles shown in the following reference is yet to be achieved. Spectral data of monolayers or multi-layers of colloidal crystals have not been reported (see H. Wang, X. Li, H. Nakamura, M. Miyazaki, H. Maeda, “Continuous Particle Self-Arrangement in a Long Microcapillary” Adv. Mater. 2002, 14, 1662).
Microsphere monolayer coatings in capillaries might become interesting for gas chromatographic application and catalytic purposes, but they cannot be applied in liquid chromatography applications, which require continuous packed phases. This problem has been overcome by the invention disclosed herein.
Colloidal crystallization and banding of microspheres in capillaries has been examined, but the morphology and spacing of the bands were not uniform. These structures are not suitable for spectroscopy of stop-bands or chromatography. The height of the microsphere dispersion column is solely determined by the contact angle and inner diameter of the employed capillary. Crystallization is induced by solvent evaporation at elevated temperatures (see M. Abkarian, J. Nunes, H. A. Stone, “Colloidal Crystallization and Banding in a Cylindrical Geometry”, J. Am. Chem. Soc. 2004, 126, 5978).
The spacing of the bands is strongly dependent upon the growth conditions, which leads to irregular spacing and banded (striped) structures if not meticulously controlled. The micromolding in capillaries (MIMIC) technique has been used to confine the nucleation and growth of photonic colloidal crystals to microchannels fashioned in a polydimethylsiloxane (PDMS) elastomeric stamp held in conformal contact with a planar substrate. Thus the capillary is formed by combining the PDMS channel with a flat substrate. Alternatively a channel or a groove in a substrate could be covered by a flat PDMS stamp to construct an enclosed capillary. The PDMS mold does not allow the operation of a pressure driven mobile phase. For chromatographic applications it is necessary to utilize a method which produces well-ordered colloidal crystals inside pressure-resistant capillary tubes. MIMIC delivers capillary photonic crystal structures but optical properties have never been reported for these structures, but nevertheless these structures are not suitable for separation applications (see E. Kim, Y. Xia, G. M. Whitesides, “Micromolding in Capillaries: Applications in Materials Science”, J. Am. Chem. Soc. 1996, 118, 5722).
Similar limitations apply to other related methods of the formation of ordered colloidal crystals with excellent optical properties in channels or grooves etched in templates (see S. M. Yang, H. Miguez, G. A. Ozin, G. A., “Opal circuits of light—planarized microphotonic crystal chips” Adv. Funct. Mater. 2002, 12, 425).
Colloidal particle arrays in capillaries have also been manufactured by employing capillary forces and not pressure to partially fill capillaries with a colloidal dispersion. After the capillary has been partially filled the fiber is removed from the dispersion reservoir. The length of the colloidal crystal is solely determined by the combination of capillary diameter and the dispersion concentration (see J. H. Moon, S. Kim, G.-R. Yi, Y.-H. Lee, S.-M. Yang, “Fabrication of Ordered Macroporous Cylinders by Colloidal Templating in Microcapillaries” Langmuir 2004, 20, 2033).
The above-mentioned article is also restricted to cylindrical capillaries and the optic properties for template constructs and inverse constructs have not been examined. These structures have not been infiltrated with liquids and there has not been any teaching in respect of chromatographic applications.
Monoliths History
Conventional liquid chromatography columns are uniform packings of roughly spherical porous particles. Separation of compound mixtures occurs via mass transfer of analytes into and out of diffusive particle pores. The diffusion rate of the separation is limited by the pores and their structural variance and poses a major source of band broadening in the resulting chromatogram. Smaller porous particles shorten the diffusive path length, improve mass transfer and provide better separation efficiency (see R. E. Majors, “Advances in the design of HPLC packings” LC GC North America 2000, 18, 586; R. E. Majors, “HPLC column packing design” LC GC Europe 2003, 16, 8; R. E. Majors, “A review of HPLC column packing technology” Am. Lab. 2003, 35, 46).
Driven by improvement in separation efficiency, column permeability is concomitantly decreased and thus the column back-pressure is greatly increased. These limitations are overcome by monolithic columns, employed as continuous separation beds or phases in liquid chromatography (LC) or in capillary electrochromatography (CEC). When discussing monolithic structures, the terms and concepts of particles and interstitial voids have to be replaced (see G. Rozing, “Trends in HPLC column formats—microbore, nanobore and smaller” LC GC Europe 2003, 16, 14).
A continuous porous structure has to be considered instead, consisting of through-pores or macropores and smaller mesopores. The macropores provide permeability and efficiency, while drastically reducing the pressure drop along the column. (see R. E. Majors, “Advances in the design of HPLC packings” LC GC North America 2000, 18, 586; R. E. Majors, “HPLC column packing design” LC GC Europe 2003, 16, 8; R. E. Majors, “A review of HPLC column packing technology” Am. Lab. 2003, 35, 46; G. Iberer, R. Hahn, A. Jungbauer, “Column watch: Monoliths as stationary phase for separating biopolymers—fourth-generation chromatography sorbents” LC GC North America 1999, 17, 998; R. E. Majors, LC GC North America 2001, 19, 1186; F. Svec, LC GC Europe 2003, 16, 24).
Monoliths are cast as homogeneous phases in situ and can potentially be used directly as columns. The porous structure is templated by dispersing a non-miscible monomeric component (porogen) and a polymerizable inorganic component in a mold or capillary and simultaneously initializing polymerization of both components. The polymerized phases can be removed separately resulting in a porous structure (see S. Hjertén, J.-L. Liao., R. Zhang “High-performance liquid chromatography on continuous polymer beds” J. Chromatogr. A 1989, 473, 1, 273; F. Svec, J. M. J. Fréchet, “Continuous rods of macroporous polymer as high-performance liquid chromatography separation media” Anal. Chem. 1992, 64, 820; H. Minakuchi, K. Nakanishi, N. Soga, N. Ishizuka, N. Tanaka, “Octadecylsilylated Porous Silica Rods as Separation Media for Reversed-Phase Liquid Chromatography” Anal. Chem. 1996, 68, 19, 3498). Control over pore sizes can be exerted to a certain degree and a double-pore structure has been disclosed (see N. Ishizuka, H. Minakuchi, K. Nakanishi, N. Soga, N. Tanaka, J. Chromatogr. A 1998, 797, 133). However, the size distribution of mesopores and macropores in these stationary phases is usually broad and the pore morphology is random (see H. Minakuchi, K. Nakanishi, N. Soga, N. Ishizuka, N. Tanaka, Anal. Chem. 1996, 68, 19, 3498; N. Ishizuka, H. Minakuchi, K. Nakanishi, N. Soga, N. Tanaka, J. Chromatogr. A 1998, 797, 133; N. Ishizuka, H. Minakuchi, K. Nakanishi, K. Hirao, N. Tanaka, Coll. And Surf. A 2001, 187-188, 273). So far, no monolithic separation phases have been presented or produced that have a bimodal pore structure with a size distribution as narrow and with a pore structure as ordered as the photonic crystal columns disclosed herein made of colloidal particles or photonic crystal columns produced by inverting the columns made of the colloidal particles. The use of polymeric porogens to produce monolithic separation beds in capillaries does not produce structure having sufficient 3-D periodical ordering to generate a change in photonic band gap structure upon interaction with electromagnetic radiation.
An incentive for the development of monolithic columns is the absence of a frit needed in conventional particulate columns to contain the separation medium particulates. The frit is a source of analyte spreading, which decreases separation efficiency. Its omission presents a major advantage in column technology.
The filling of capillaries with beads or microspheres usually requires the presence of a frit to contain the microparticles inside the capillary. Such frits contribute to the peak broadening in the chromatogram, due to inferior structural order, the additional diffusive path length, and/or different interaction of the analytes as stationary phase and frit are made from different materials. For bead templated porous monolithic columns the initial frits are produced by tapping one end of the capillary into a paste prepared from the utilized silica particles and a silicate solution. The resulting plug is fused in situ (see G. S. Chirica, V. T. Remcho, “Novel monolithic columns with templated porosity” Journal of Chromatography A 2001, 924, 223).
Monolithic Columns Based on Colloidal Crystal Templates
Monolithic columns with bead templated porosity have been produced in capillaries. Silica beads were packed into capillaries prior to flushing the bead array with a monomer solution. After polymerization of the monomeric components the silica beads were removed by aqueous wet etching (see G. S. Chirica, V. T. Remcho, “Novel monolithic columns with templated porosity” Journal of Chromatography A 2001, 924, 223). Despite using a template structure no photonic band structure is generated, the material is irregular and lacks the necessary periodicity of the invention disclosed herein.